Modern webpages can be generated by combining data from multiple sources in real time. Almost a limitless number of variations can be created for an individual webpage making quality control very difficult. For example, a table on a webpage could be populated with data drawn from one or more knowledge bases in response to a query. Images and advertisements could be presented from other sources. A search engine might generate search results in multiple formats. In each case, a mismatch between source data and presentation instructions can cause display abnormalities on the webpage. For example, a table could be presented with multiple blank fields because the underlying database does not have data for certain fields. In another example, too much or too little space could be allocated to a field in a table. Catching and fixing these errors remains a challenge.